1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof and more particularly to an improvement on a SOC (System On Chip) that has a logic (logic circuit) and a DRAM (Dynamic Random Access Memory) together on one and the same substrate.
2. Description of the Related Art
In a general purpose DRAM (having, on one chip, only a memory section and an adjacent circuit section which comprises a decoder, a sense amplifier, an I/O (input/output) circuit and the like), hold characteristics of memory cells have been, hitherto, regarded as the matter of great importance so that it has become a common practice to set the dopant concentration in the source-drain regions of cell transistors lower than that in the adjacent circuit section for the purpose of achieving the suppression of the junction leakage current.
Meanwhile, in recent years, demands that image processing using computer graphics should be made at a higher speed have been growing. Accordingly, there has been proposed a DRAM-incorporated logic chip as shown in FIG. 9, a so-called SOC 31 which has a DRAM section 32 comprising memory cells 33 and adjacent circuits 34 such as a decoder and a sense amplifier, a logical operation section (a logic section) 35 where graphic processing is carried out at a high speed, and, in addition, an I/O section 36 which connects these DRAM and logic sections with an external circuit, all together on one chip.
In the DRAM section thereof, to suppress the short channel effect and relax the centralization of the drain field, the dopant concentration in the source-drain (S/D) should be, hitherto, set low. This makes the junction in the S/D regions shallow and, as has been pointed out, the silicidation therein may bring about an increase in the leakage current. In light of this problem, several techniques of making silicidation in the DRAM section have been proposed
For example, Japanese Patent Application Laid-open No. 97649/1999 discloses the methods using the following SOC structures: that is, (1) a structure (first embodiment) wherein, while the dopant concentration in S/D regions of a memory cell region in a DRAM section is set low to make the leakage small, dopants are implanted into S/D regions of an adjacent circuit region to a high concentration, and besides silicide layers are formed on gate surfaces and the surfaces of active regions, and thereby high-speed operations become attainable; (2) a structure (second embodiment) wherein, in a DRAM cell section, S/D regions (low dopant-concentration regions) with which capacitor contacts are connected are set as non-silicide regions, and silicide layers are formed only on the surfaces of S/D regions (low dopant-concentration regions) with which bit line contacts are connected and on the gate surfaces of cell transistors, whereby the interconnection resistance and the contact resistance are reduced, and reading and writing operations at a higher speed are made attainable; and (3) a structure (third embodiment) wherein, in a DRAM memory cell section, portions of a TEOS (Tetra-Ethyl-Ortho-Silicate) oxide film that is first laid at the time of formation of transistor sidewalls are selectively left at the edge of LOCOS (Local Oxidation of Silicon), or in the vicinity of bird's beaks, and thereafter silicide layers are formed on the entire surface of the DRAM memory cell section but said portions, in the same way as for cell gates and a logic section, and thereby reading and writing operations at a higher speed are made attainable, while the junction leakage is kept low. As an example, the third embodiment in said publication is now described in detail with reference to a schematic cross-sectional view of a memory cell section shown in FIG. 10. A semiconductor substrate 41 is isolated into elements by isolation oxide films 42 and a memory cell transistor is formed between these films. Silicide layers 49 are formed on the surfaces of the gate electrodes 48 and in regions where a contact 52a connecting with a bit line 51 and contacts 52b connecting with storage nodes 53 are in contact with the semiconductor substrate 41, respectively. These silicide layers lying on the substrate are formed within low dopant-concentration regions 47. At the edge sections of the isolation oxide films 42, there are formed TEOS oxide films 50, protecting these edge sections from silicidation, so that silicide layers may not be formed beneath the isolation oxide films 42 or the junction leakage may not be generated around the edges thereof. In the drawing, 43-45 each represents an interlayer insulating film; 46, gate oxide films; 54, a dielectric film and 55, a cell plate.
Further, Japanese Patent Application Laid-open No. 17129/1999 discloses a structure wherein, after S/D regions of transistors in a DRAM cell section and a logic section are formed in separate steps, respectively, and contact-holes that reach S/D regions of transistors for cell selection are formed, silicide layers are formed on the surfaces of the S/D regions through said contact-holes so that conductive films are buried in the contact-holes. In this instance, silicide layers are also formed in the S/D regions of transistors in the logic section but this silicidation is carried out in a different step from the one for silicidation of the S/D regions of transistors for cell selection. Further, it is therein mentioned that silicidation can be also applied to the gate surfaces in another step.
In any of these conventional techniques, the S/D regions of DRAM cell transistors are still formed to have a low dopant concentration, while the S/D regions of transistors in the logic section, a high dopant concentration. In other words, a SOC structure of this sort is designed under the concept that a high-speed logic is made incorporated into an existing DRAM. This assumes fabrication of the DRAM section and the logic section in separate steps and, thus, a reduction of the production cost cannot be much expected to happen. However, the SOC is, by nature, designed and manufactured for each system application. In comparison with the general purpose DRAM, a small quantity and a large diversity characterize its production, and, therefore, the cost reduction has a particular significance.